Re: [Xen-devel] [PATCH] x86/tss: Fix clang build following c/s 7888440625

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.08.2019 14:01, Andrew Cooper wrote:
> Clang-3.5 from Debian Jessie fails with:
>
>    smpboot.c:829:29: error: statement expression not allowed at file scope
>            BUILD_BUG_ON(sizeof(this_cpu(tss_page)) != PAGE_SIZE);
>                                ^
>    /local/xen.git/xen/include/asm/percpu.h:14:7: note: expanded from macro
>            'this_cpu'
>        (*RELOC_HIDE(&per_cpu__##var, get_cpu_info()->per_cpu_offset))
>          ^
>    /local/xen.git/xen/include/xen/compiler.h:98:3: note: expanded from macro
>            'RELOC_HIDE'
>      ({ unsigned long __ptr;                       \
>      ^
>    /local/xen.git/xen/include/xen/lib.h:26:53: note: expanded from macro
>            'BUILD_BUG_ON'
>    #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
>                                                        ^
>    /local/xen.git/xen/include/xen/lib.h:25:57: note: expanded from macro
>            'BUILD_BUG_ON_ZERO'
>    #define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
>                                                            ^
>    1 error generated.
>    /local/xen.git/xen/Rules.mk:202: recipe for target 'smpboot.o' failed
>
> This is obviously a compiler bug because the BUILD_BUG_ON() is not at file
> scope.  However, it can be worked around by using a local variable.
It can be worked around, yes, but the result is not identical:

> --- a/xen/arch/x86/smpboot.c
> +++ b/xen/arch/x86/smpboot.c
> @@ -826,9 +826,11 @@ static int setup_cpu_root_pgt(unsigned int cpu)
>           rc = clone_mapping(idt_tables[cpu], rpt);
>       if ( !rc )
>       {
> -        BUILD_BUG_ON(sizeof(this_cpu(tss_page)) != PAGE_SIZE);
> +        struct tss_page *this_tss = &per_cpu(tss_page, cpu);
Whatever type is compatible with struct tss_page * will now be
permitted on the rhs. Hence ...

> -        rc = clone_mapping(&per_cpu(tss_page, cpu).tss, rpt);
> +        BUILD_BUG_ON(sizeof(*this_tss) != PAGE_SIZE);
... this sizeof() is not necessarily checking the size of the
per-CPU variable. But anyway - the change has been committed
already, and the chances of this going wrong are low enough, so
please don't treat this as an objection or revert request.

Jan

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